I don't know if anyone's brought this specific idea up before but I've uncovered a little more about the rules behind aging in Soul Society. It seems only non-Shinigami with spirit powers age. I'm not sure, but its possible that non-Shinigami age at the same rate as normal humans. Notice the future shinigami in the academy seemed younger than they do now. Since most people don't have spirit powers, they don't use up energy to require a metabolism to age. But at the same time, Shinigami don't seem to age, or do so at a much much slower rate than nonshinigami who age. I think it may be Shinigami powers that stop or severely cut down aging. While the first one half seems obvious(that people with spirit powers age), the second is more subtle. If you notice, Rukia seemed to stop aging when she got her powers. She killed Kaien when Ganju was a little boy, around 6ish looking, yet Ganju looks mid20s-early30s right now. At the same time, Rukia doesn't look a year older. This would explain why Rukia is 10x older than Ichigo, not because she aged 10x slower, but because she got her powers when she became Ichigo's age. That explains why she only seems younger in flashbacks before she got her powers along with everyone else. I hope I explained it clearly enough.